1. Field of the Invention
This invention relates to a spark plug of which precious metal chip disposed on the side of the tip portion of the center electrode opposes the ground electrode thereof, and a method of producing the same.
2. Description of Related Art
Conventionally, for example, a spark plug is proposed of which a side of the tip portion of the center electrode opposes the ground electrode thereof, and which generates a spark discharge in the gap between the side and the ground electrode. A precious metal portion with superior resistance to erosion is provided on the side of the center electrode.
As the above-mentioned precious metal portion, for example, an alloy of platinum, iridium, and nickel (Pt--Ir--Ni) that can withstand severe conditions has been employed.
The above-mentioned precious metal portion is formed in an annular, cap-shaped, or similar configuration, and is engaged and bonded to the tip of the center electrode.
Metal material such as a heat-resistant nickel alloy is employed for the center electrode.
However, the above-mentioned conventional spark plug presents the following problem.
Briefly, the precious metal portion which suppresses erosion of the center electrode surrounds the entire side circumference of the center electrode, but the area which opposes the ground electrode is only a fraction of the total precious metal portion. For this reason, the precious metal portion on other areas serves no functional purpose, and is discarded along with the spark plug at the end of its service life.
The precious metal portion having unused areas is discarded. This is not desirable from the standpoint of resource conservation. Production cost is also increased.
In addition, when precious metal portion is employed in a cap-shaped configuration, the precious metal on the top surface of the cap is completely wasted.
The precious metal portion described above also has a smaller coefficient of linear expansion than the metal material of the center electrode.
For this reason, when an annular or cap-shaped precious metal portion is bonded to the center electrode, the precious metal portion is subjected to thermal stress due to the difference of linear expansion coefficient between the center electrode and the precious metal, and major effects are applied. Because of this, there is danger of cracks occurring in the precious metal portion.
In severe cases, this may lead to peeling or loss of the precious metal portion. The possibility also exists of peeled fragments of precious metal portion bridging the spark gap. For this reason there is the problem of an inability to withstand prolonged service life.
In this regard, in order to avoid the above-mentioned thermal stress, there exists, for example, a method whereby the above-mentioned precious metal portion is inserted into the tip of the center electrode and bonded by brazing (Japanese Patent Publication laid-open No. 4-92383.
With this method, the thermal stress of the precious metal portion and the center electrode is alleviated because of a layer of the brazing material is formed between the precious metal portion and the center electrode.
With this brazing method, however, because of reasons such as the high expense of the brazing material used and the large number of brazing processes, the production costs of the spark plug are increased.
Another method exists whereby the center electrode is formed by extrusion after an annular precious metal portion is engaged to the tip of the center electrode, thereby providing an annular precious metal portion on the side of the top of the center electrode (Japanese Examined Patent Publication No. 62-31797).
In order to achieve the practical application of this method, a method is employed to alleviate thermal stress by making the linear expansion coefficient of the precious metal portion approach the linear expansion coefficient of the center electrode.
For example, in the case where a platinum-iridium-nickel alloy is used as the precious metal portion and a nickel alloy is used as the center electrode, the linear expansion coefficient of the precious metal portion can be increased and brought closer to the linear expansion coefficient of the center electrode by increasing the nickel composition of the alloy for the precious metal portion.
However, when the alloyed composition (nickel) is increased in order to increase the linear expansion coefficient of thermal expansion of the precious metal portion, the melting point of the precious metal portion decreases, and there is danger of the amount of erosion due to spark discharge increasing. Because of this,in order to prolong the service life of the precious metal portion, extremely large amounts of precious metal must be utilized. Consequently, cost is increased.